Electrolytic fluid treatment systems are widely used to, for example, remove impurities and contaminants from fluids. In such systems, the fluid to be treated is passed between one or more pairs of electrodes. An electric potential applied to the electrodes establishes an electric current between the electrodes. As a result, impurities in the fluid migrate and adhere to the electrodes, biological materials in the fluid are killed, and the fluid's chemical composition may be altered.
One fluid that is commonly processed by electrolytic fluid treatment systems is water. The electrolytic treatment of water is, however, complicated by the widely varying water characteristics encountered from one water source to another. In that regard, the resistivity of water, which is inversely proportional to conductivity, commonly varies over a range extending from 30 to 1400 ohm-meter. Such resistivity variations may significantly alter the performance of an electrolytic filter system.
More particularly, the interelectrode resistance is dependent upon the resistivity of the water flowing between the electrodes. With a fixed electric potential applied to the electrodes, current flow between the electrodes will vary in inverse proportion to the water's resistance. If water resistivity is relatively high, the current may be too low to achieve the desired treatment of the water. On the other hand, if water resistivity is relatively low, the current may be so high as to damage or otherwise decrease the life of system components.
A variety of different systems have been developed that attempt to accommodate such variations in water resistivity. For example, circuits have been developed to expose water purification and ion generation systems to relatively constant load resistances, regardless of variations in water resistivity. In that regard, U.S. Pat. No. 4,769,119 (Grundler) discloses a water ionizing device that includes several electrodes. If the resistivity of the water being ionized is relatively low, a relatively high resistance is introduced in series with the electrodes. On the other hand, if the water's resistivity is relatively high, a relatively low resistance is introduced in series with the electrodes. In either case, by keeping the system's total resistive load constant, a constant current flow is maintained between the electrodes.
U.S. Pat. No. 4,986,906 (Dadisman) describes another variation of this approach. The Dadisman water purification system includes a constant current control circuit in which changes in water resistance cause opposing changes in the effective resistance of a field-effect transistor (FET) included in the circuit. These changes in FET resistance offset the changes in water resistance, allowing the current to be kept substantially constant.
Unfortunately, the approaches taken by Grundler and Dadisman have certain limitations. In that regard, the Grundler and Dadisman circuits both increase circuit resistance to offset decreases in water resistance. As a result, energy is dissipated in circuit components rather than being used to treat water, making the circuits relatively inefficient. In addition, the Grundler and Dadisman circuits are both relatively complex.
An alternative method of handling variations in water resistivity is to provide an electronic control circuit that allows water purification and ion generation systems to maintain constant current flows, substantially independent of variations in water resistivity. In that regard, U.S. Pat. No. 4,119,520 (Paschakarnis et al.) discloses a water purification unit that includes such a current control circuit. The current to be controlled flows through a resistor, as well as between the electrodes. A differential amplifier and transistor cooperatively control the current by keeping the voltage drop across the resistor equal to the reference potential across a diode. As a result, the current flowing between the electrodes is kept constant.
Similarly, U.S. Pat. No. 5,055,170 (Saito) discloses an ionic water generator that accounts for variations in water resistivity. In that regard, the system employs a central processing unit that calculates the appropriate voltage to be applied to the electrodes for the water being processed. This voltage is computed by multiplying some voltage corresponding to the desired ion concentration by a factor equal to the resistance of the water actually being processed divided by the resistance of some reference water.
As will be appreciated, the Paschakarnis et al. and Saito systems exhibit several shortcomings. First, the control circuits of both systems are relatively complex. Because the Paschakarnis et al. circuit introduces an additional resistance into the current path, it is also relatively inefficient. The Saito circuit, in turn, disadvantageously requires reference measurements to be made for subsequent use in controlling the voltage applied to the electrodes.
Another circuit for controlling an ion generator in a water purification system is disclosed in U.S. Pat. No. 4,734,176 (Zemba, Jr.). The circuit controls the duty-cycle of energy applied to the generator to achieve the desired level of purification for various applications and water conditions. In that regard, an operator apparently evaluates water conditions and then manually adjusts the control circuit to effect a desired change in duty cycle.
Like the other circuits described above, the Zemba, Jr. arrangement has certain limitations. For example, the ability of the Zemba, Jr. circuit to handle variations in water resistivity is not discussed and is uncertain. Also, because the circuit does not automatically respond to changing water conditions, it may fail to achieve the desired regulation in many instances.
Turning now to another problem experienced in the electrolytic treatment of fluids, conventional electrolytic fluid treatment systems typically do not perform equally well in removing impurities, killing biological materials, and altering the fluid's chemical composition. At best, existing systems achieve one of the desired objectives relatively well, while exhibiting compromised performance with respect to the other objectives. More particularly, most such systems fail even to differentiate between these various objectives, much less achieve them fully and simultaneously.
One final problem encountered in the electrolytic treatment of fluids is the limited ability of conventional systems to provide large quantities of energy to the fluid over brief intervals in an efficient manner. For example, while some systems may be suitable for passing relatively low currents through the fluid during short intervals, they are typically unable to apply higher currents to the fluid. Alternatively, while some systems are able to provide high currents to the fluid quickly, they are relatively inefficient.
In view of these observations, it would be desirable to provide a control circuit suitable for controlling the operation of an electrolytic fluid treatment system substantially independent of variations in the resistivity of fluid treated by the system. It would also be desirable to provide a control circuit that allows several aspects of the system's performance to be optimized, without undue circuit complexity, inefficiency, or operator intervention and that is able to provide relatively large quantities of energy to the fluid quickly and efficiently.